(1) Field of the Invention
The present invention relates to a multi-layer electrodeposition coating film-forming method which is capable of forming a multi-layer electrodeposition coating film showing good properties in weather resistance, corrosion resistance and finished appearance, and in which an electrodeposition coating composition shows good coating composition stability.
(2) Description of the Background Art
The cationic electrodeposition coating composition is widely used in an automobile body, automobile parts, household appliances, equipments, and the like, in that the cationic electrodeposition coating composition can form a coating film showing good properties in weather resistance, corrosion resistance and finished appearance.
The above features of the cationic electrodeposition coating composition resulted an increased application of the electrodeposition coating composition to a coating product formed by a sole electrodeposition coating (hereinafter referred to as a one coat electrodeposition coating), and demands on a cationic electrodeposition coating film showing good properties in both weather resistance and corrosion resistance.
A multi-layer electrodeposition coating film-forming method capable of forming a multi-layer electrodeposition film showing good properties in weather resistance and corrosion resistance as known in the art may include ones disclosed, for example, in Japanese Patent Application Laid-Open Nos. 174277/87, 333528/96, 292131/98, 345394/00, 234116/01 and the like, in which a multi-layer electrodeposition coating film-forming method to result a layer separation in the coating film by having a difference between a solubility parameter (SP) value of the acrylic resin showing good weather resistance and a solubility parameter (SP) value of the epoxy resin showing good corrosion resistance is disclosed.
In the case where a cationic electrodeposition coating composition contains at least two resins having respective solubility parameters different from each other, a wet coating film formed by the electrodeposition coating has such a hardness as not to have a finger mark thereon, so that a shortened heat-curing time from the standpoints of shortening of coating steps an energy savings makes difficult a migration of the acrylic resin to a surface layer of the coating film and a migration of the epoxy resin to an interface with the steel plate only by hot air flow on heat-curing. Particularly, in the case of a thick parts such as a wheel, under parts and the like, a shortened heat-curing time and difficulties in temperature rise may form a coating film comprising a mixed layer of an acrylic resin layer and an epoxy resin layer, resulting in obtaining a coating film showing unsatisfactory properties both weather resistance and corrosion resistance.
Further, the cationic electrodeposition coating composition in the coating line may be subjected to prolonged shears or loads, for example, circulation of the coating composition by a pump, ultrafiltration (UT), fine filtration, vaporization of solvent, inclusion of admixtures such as a chemical solution, degreasing solution and the like, showering with a recovered washing water, settling and diffusion of the coating composition in the bath, and the like.
A large difference in the solubility parameter between resins contained in the cationic electrodeposition coating composition under the above conditions produce various problems, for example, coating composition, stability problem, reduction in finished appearance, increase of filter exchange frequency due to blockage of fine filtration, increase of a taken-out amount of the coating composition due to reduction of ultrafilter-permeating amount, increase of cleaning frequency due to settling of the coating composition, and the like.
Such being the case, a multi-layer electrodeposition coating film-forming method capable of making possible an easy layer separation with a cationic electrodeposition coating composition showing good stability is highly demanded.